This invention relates to a homogeneous, liquid palladium-containing catalyst preparation which is soluble in apolar media.
Transition metal catalysts based on palladium are known for the hydrogenation of aliphatically unsaturated carbon-carbon bonds and for the reduction of various functional organic groups, for example aldehyde, keto and nitro groups, with hydrogen. The palladium is normally used in heterogeneous form on a support material. Example are palladium on active carbon, barium sulfate or calcium carbonate, as described in Rylander ("Catalytic Hydrogenation in Organic Synthesis", Academic Press; New York, 1979, pages 1-22, 33-34).
Soluble palladium-containing hydrogenation catalysts are less well-known. Stern and Maples (Journal of Catalysis, Vol. 27, (1972), 120-133) describe the hydrogenation of unsaturated organic compounds with Pd(O) complexes of the (L).sub.3 Pd.sub.2 or (L)PdCl.sub.2 type (L=Ph.sub.2 P-(CH.sub.2).sub.n -PPh.sub.2). Sisak, Jablonkai and Ungvary (Acta Chimica Adademiae Scientiarum Hungaricae, Vol. 103 (1980), 33-42) describe the hydrogenation of dienes and alkynes with (DMF).sub.2 PdCl.sub.2 (DMF=dimethyl formamide). Other catalysts of the (L).sub.2 PdCl.sub.2 type (L=PPh.sub.3 or dimethyl sulfoxide) are also mentioned. These catalysts are preferably used in polar organic solvents, such as DMF.
However, these catalysts have disadvantages. The palladium(O) compounds are sensitive to atmospheric oxygen wile the palladium(II) compounds are not sufficiently soluble for use in apolar media.
Important apolar media are apolar liquids, such as hexane, tolune or diethyl ether and cyclic, linear and/or branched siloxanes. Solubility in apolar liquids where hydrogenation is carried out in the absence of solvents is also desirable. For example, adequate solubility is also necessary in the hydrogenation of vinyl-containing siloxanes or in the production of hydrogen-absorbing polysiloxane preparations, as described in GB 2,167,424 A.
Accordingly, the object of the present invention was to provide highly effective and soluble catalysts for hydrogenation in apolar media.